The objective of this study is to obtain knowledge concerning the functional organization of mammalian magnocellular neurosecretory neurons in the supraoptic nucleus, and of neuronal mechanisms controlling their activity. Mature sheep will be surgically prepared under halothane anesthesia with a cranial platform-cylinder arrangement and pituitary stimulating electrodes in preparation for subsequent microelectrode recordings from single antidromically identified, supraoptic units. Spontaneous firing patterns will be characterized in the unanesthetized animal and their response observed to vaginal distention (previously demonstrated to release oxytocin in sheep) and to combinations of carefully quantitated changes in plasma osmolality and blood volume (previously demonstrated to release antidiuretic hormone in sheep). Behavioral parameters (EEG, eye movement, and body movement), left atrial pressure, and mean arterial blood pressure will be monitored for possible correlation with unit response periods. Firing patterns of a single unit during several periods of control, stimulus "forcings", and behavioral states will be analyzed by an IBM 360-65 computer with a program designed to compute mean firing rate, interspike interval mean, standard deviation, coefficients of variation, histogram of any desired order, and the spike train autocorrelogram. The functional significance of spontaneous firing patterns (silent, continuously active, and low frequency bursting), previously associated with these neurosecretory cells in the unanesthetized monkey and sheep, will be examined. It will be determined if they are a reflection of certain sensory input connections and associated with a specific hormone secretion, and/or if they are a function of varying recruitment thresholds to stimulus intensity. The convergence of sensory input, distribution of sensory specific neurosecretory neurons, and dynamic characteristics of the osmoreceptor-neuroendocrine cell system will also be determined.